Research report 1998

This is our 5th web-based research report since we went
'online' in 1994. Since then, our website has attracted a lot of attention from all
over the world and meanwhile serves as an information source not only for our physics
colleagues and interested students but also for those out there, who simply want
to know on which subjects their 'tax Euros' are working. We appreciate the tremendous
activity on our server and do encourage everybody to come by and stay for a while!

For our group, the last year was an extremely sucessful one:

In total, the group publishedmore than 35 scientific papers, and presented 15 invited talks on international conferences and workshops on various subjects and most
importantly, 12 students finished their Diplomaor PhD thesis. Dr. David Wharam
accepted a professorship at the University of Tübingen - we wish him all
the best! We had the pleasure to welcome a new member of our scientific staff, Dr. Robert Blick,who brought in new ideas and techniques, Achim Wixforth
received the 'Walter-Schottky-Prize for Solid State Research', and finally, Jörg
Kotthaus could be convinced to stay with us and hence to ensure the continuity of
our daily work. Many visitors from abroad choose to spend some time with us or came
by to give seminar talks.

Finally, we would like to encourage the readers of this (although
somewhat delayed) report to contact us and to share their comments and ideas with
us.....

OVERVIEW

Modern semiconductor technology nowadays combines more than ten
million different transistors on a single chip barely as big as a thumbnail to form
an extraordinary complex and sophisticated circuit. Following Moore's law, this very
large scale integration will proceed over roughly the next ten years until a single
element on a chip will be scaled down to less than about 50 nm. This typical dimension
of a single device, however, represents a barrier, beyond which the basic operation
of an electronic device starts to be based on fundamentally different mechanisms
as compared to the conventional ones.

In a classical silicon MOSFET, for example, the principle of operation
is based upon the statistical motion of about 10'000 electrons per square micron,
whose number may be varied by an external electrode via electric fields. This movement
takes place close to the relatively rough silicon/silicon dioxide interface and is
described by diffusive processes, similar to the Brown's motion of molecules.

If, however, the dimension of a device becomes comparable or even
smaller than the typical distance between two scattering events, the electrons start
to move ballistically, like the balls on a billiard table. Moreover, at these
small sizes, the number of electrons within a single device starts to approach one.
For even smaller devices, their size becomes comparable to the wavelength of the
electrons themselves - typically some ten nanometers in this case: The description
of the electrons behaving like little charged spheres starts to fail and to require
for a quantum mechanical formulation of the device.

In our group, we investigate the electronic, electrooptical,
and electromechanical properties of specially tailored semiconductor structures with
typical dimensions of the order or less than 100 nm. Recently, we also started to
process and investigate mechanical systems like resonators and oscillators on the
nanometer scale. Our goal is the detailed understanding of the new physical phenomena
associated with a dramatic reduction of size, to explore new grounds for future device
applications, and to be prepared for the day when nano-electronics will take
over the role of micro-electronicsand micro
or nano-mechanics will open new routes to the tiny ultra small universe!

- to boldly go where no person
has ever gone before !

The research in our group is based on three fundamental prerequisites
:

Nanotechnology

Sophisticated electronic and optical experimental
techniques

Quantum mechanical concepts and analysis

Starting from suited semiconductor layered systems, we first have to prepare
the desired structures with lateral nanometer size dimensions. We use and develop
different nanotechnologies that enable us to scale down the size of our structures
to the size of the electronic wavelength. For this purpose, our nanotechnology labs
are located in a dust free cleanroom area containing modern semiconductor processing
equipment.

As we're always trying to be internationally competitive, we set
up a large number of international co-operations with partners being specialized
in the epitaxial growth of our high quality starting material. Meanwhile, our nanotechnological
techniques are also transferred to different disciplines of leading edge research
resulting in newly developed collaborations with highly qualified specialists in
x-ray analysis, polymer physics and biophysics.

Secondly, we constantly develop and apply sensitive experimental techniques
which enable us to chararcterize and to investigate the electronic and optical properties
of our nanometer scale samples over the whole spectral range starting from DC over
the microwave and infrared regime, the visible spectrum up to UV. At the same time,
we are equipped with facilities allowing fo extremely low temperatures and high magnetic
fields - invaluable tools for the detailed understanding of the quantum mechanic
phenomena in our devices.

A third prerequisite for our research is a detailed and fundamental theoretical
analysis and understanding of nanophysics. Together with many theoretical groups
and in a very fruitful atmosphere of collaboration, we try to develop new theories
and techniques helping us to understand or to predict the many fascinating effects
that we are constantly facing. This is in particular important, as we are not studying
systems already existing in nature but try to artificially tailor small pieces
of this nature to behave in a desired fashion.

Summary of the different research topics

Hybrid-based acoustic charge transport device operating at room temperature. The
device consists of a thin GaAs/InGaAs semiconductor quantum layer containing a quantum
well with a two-dimensional electron system that is transferred onto a strongly piezoelectric
substrate. On this substrate, a surface wave is propagating. The interaction between
the large amplitude wave and the electron system leads to new, nonlinear effects
and to acoustic charge transport across the mm long sample.

Surface acoustic waves are modes of elastic energy which can propagate
on the surface of different materials. If the substrate is piezoelectric, those waves
are accompanied by electric fields which then propagate at the speed of sound. The
electric fields of the wave can couple to the mobile carriers within a semiconductor
structure and modify its electronic and elastic properties. By measuring the attenuation
of the wave and the renormalization of the sound velocity we can, for instance, extract
information on the dynamic conductivity of the electron system. We also investigate
the possibility to use a SAW for a dynamical lateral potential modulation and we
investigate the influence of a SAW onto the optical properties of an electron system.
Our experiments presently cover the frequency range between 100 MHz and 6 GHz, corresponding
to surface acoustic wavelengths between 30 mum and 500 nm, respectively.

The SAW - related research in 1998 was again governed by two different
topics:

Our recent progress in fabricating high frequency and multi-frequency
SAW transducers enables us to expand our SAW transport studies well into the GHz
regime. The possibility to generate multiple frequencies on a single device allows
for a detailed investigation of the SAW - 2DES interaction as a function of the frequency.
On standard semiconductor heterojunctions, however, the strength of this interaction
is rather weak. For this reason, we use hybrid structures consisting of a strong piezoelectric
(LiNbO3) the active semiconductor layer structure. Here, a thin layer
of the semiconductor structure containing the active heterojunction is removed from
its substrate and tranferred onto a strongly piezoelectric host substrate.

The SAW - 2DES interaction can be enlarged
by two orders of magnitude as compared to the monolithic
case. This interaction is now strong enough to become technologically very attractive.
Room temperature operation is also possible. Together with the Siemens research lab, we are presently developping
new concepts for a technological exploitation of this approach. apart from technologically
challenging projects, the hybridization technique also offers nice possibiliteis
to explore the interaction between low-dimensional electron systems and surface acoustic
waves with very large amplitudes. Here, we focussed on the investigation of nonlinear
effects that arise under these conditions. For example, we could show that this nonlinear
interaction leads to a strong decrease of the electronic absorption that is linked
to a separation of a formally homogenous carrier density in the electronic system
into well separated stripes riding the wave.

Our maior activities, however, concentrated on the investiagtion
of the influence of a SAW on the optical properties of a semiconductor quantum well.
Here, we opened a completely new and exciting area of research and attracted a tremendous
national and international interest. We observe a strong influence of the SAW on the optical properties of a quantum well
. The photoluminescence of the quantum well can be completely
quenched under the influence of the SAW. Moreover, we show that the SAW can act as
a "Photon Conveyor Belt", where optical
signals can be stored by the SAW in the semiconductor and may be re-assembled into
light after very long delay times and at a remote location of the sample!

Also, the possibility to deliberately pump a self-assembled quantum dot on the surface
of a semiconductor quantum well has been investigated both experimentally as well
as theoretically. In collaboration with the group of Prof. Fritz Haake (University of Essen), we have
proposed a novel scheme for the generation of a single photon source using surface
acoustic waves. A non-classical light might evolve from these studies, that we have
also started to perform experimentally.

SAW-induced lateral potential modulations in hybrid systems could
be shown to also act as an effective potential modulation for a quasi two-dimensional
electron system under quantum conditions. For small wavelengths, we were able to
observe commensurability oscillations in the acoustoelectric effect in such structures.
Here, the classical cyclotron diameter equals the wavelength of the SAW, leading
to pronounced oscillations in the acoustoelectric voltage across the semiconductor
sample.

A.2. Electron Transport in Antidot-Lattices

"Antidot" lattices can be considered the complementary
structure to quantum dot lattices: While in the latter, an array of isolated islands
of electrons is created, in the former an array of small voids is cut out of a two-dimensional
electron gas. In the past, a number of novel effects have been observed both in classical
as well as quantum transport, which result from the interplay between different length
scales in antidot-lattices (lattice period, magnetic length, Fermi-wavelength). Whereas
commonly circular voids are patterned, we have created lattices of cross-shaped antidots.
Such lateral superlattices can equally be regarded as antidot arrays, as two-dimensional
arrays of quantum point contacts or as arrays of coupled quantum dots. They are therefore
promising candidates to study the the transition between different types of quantization,
e.g., between the quantized conduction in 1-dimensional channels at zero magnetic
field and the quantum Hall effect in 2-dimensional electron gases at high fields.
In the antidot picture, the complex unit cell is expected to lead to more complex
carrier dynamics, compared to the well-investigated case of simple (round) antidots.
Indeed, a number of novel structures in the magnetotransport properties of "+"-shaped
antidots have been observed. Puzzling at first, the origin of these feature could
be clarified by a detailed analysis of the quasi-classical ballistic electron trajectories,
which was performed in collaboration with the Regensburg University.

A.3. Ballistic Rectifier

Furthermore, antidot lattices have been investigated where the
triangular shape of the basis breaks the left-right symmetry of the electron system.
In low frequency transport these samples exhibit maxima in the magnetotransport at
a magnetic field where the cyclotron diameter equals half the lattice period. This
is caused by so-called skipping orbits and shows that it is indeed possible to transfer
triangular shapes with straight edges into the two-dimensional electron gas. In high-frequency
experiments we observe lateral photo-voltages which closely reflect the features
in low-frequency transport caused by the presence of the antidots. This might be
due to the fact that the broken symmetry of the antidot-lattice leads to a rectification
of the high-frequency radiation.

To further investigate the influence of symmetry-breaking on transport
properties in mesoscopic semiconductor structures, we have fabricated cross junctions
with an embedded triangular antidot. As seen in the figure, a simple picture of ballistic
electron transport predicts unusual transport behaviour in such a device: Independent
of current direction, electrons ejected out of the narrow channels (S and D) will
be reflected from the edges of the antidot towards the lower (L) channel. This way,
a voltage is induced between the L and the U channel which does NOT change sign,
when the input current is reversed. Indeed, a clear voltage was observed experimentally
on such samples, which exhibits the above mentioned unusual symmetry relations.

A.4. Coulomb Blockade Phenomena in Quantum Dots

If very small islands of electrons are isolated from a two-dimensional
electron gas via tunnel barriers, the charging energy needed to add additional electrons
to the island can be higher than the thermal energy available. In such a case, which
usually requires very low temperatures, electron transport through such a 'quantum
dot' is blocked.This so-called 'Coulomb blockade' can be raised by tuning the islandís
energy with an additional plunger gate (see figure). A plunger gate sweep results
in quasi-periodic conductance oscillations.The difference between adjacent conductance maxima is the ëaddition
energyí. In the simple ëconstant interactioní model this energy
can be expressed as the sum of a classical capacitive charging term and a quantum-mechanical
single particle energy level spacing.We have performed Coulomb blockade measurements on a semiconductor
quantum dot fabricated in a GaAs-AlGaAs-heterostructure and investigated the fluctuations
and the distribution of the conductance peak spacings (9-97) obtained from these
measurements. The statistical properties were compared to the predictions of random
matrix theory (RMT) as this theory provides a very good description of the spectra
of many complex systems.It was found that the fluctuations in the peak spacings are larger
than expected from RMT and the distribution of the spacings resembles a Gaussian
rather than a Wigner-like probability distribution. This indicates that the peak
spacings are strongly affected by the electronic interactions on the dot. However,
both constant interaction model and RMT deal with single particle spectra. Until
now, there is still put considerable theoretical effort on the understanding of the
peak spacing distributions.

REM micrograph of an electrostatically defined quantum dot. Two tunnel contacts
provide access to the transport properties of the dot, which can be controlled by
using a central plunger gate. The lithograpically defined dot area is about 0.5 *
0.5 microns.

A.5. Epitaxial InGaAs surface stressors

With advances in miniaturization of semiconductor structures, the interplay
between mechanical stress and electronic properties becomes more and more important.
On the one hand, mechanical stress can limit the performance of small scale electronic
devices. On the other hand, a controlled application of stress can be used to tailor
the optical and electronic properties of semiconductor structures beyond common patterning
techniques, such as etching or electrostatic modulation. We have used a novel type
of a coherently strained stressor structure to create a one-dimensional periodic
potential in the two-dimensional electron gas at a AlGaAs/ GaAs heterointerface.
Contrary to conventional (e.g. evaporated or sputtered) strained structures, epitaxial
stressors offer the possibility to accurately control the amount of stress during
fabrication and allow for a precise theoretical analysis, starting from the well-known
material parameters and matching conditions. From the analysis of the magnetotransport
we can determine the Fourier-coefficients of the conduction band modulation. This
reveals that in contrast to conventional electrostatic patterning, "hard"
potential modulation with dominant contributions of higher harmonics can be achieved.
With the strong contributions of the higher harmonics, already at a 600 nm periodic
superlattice Fourier components with periods in the range of 90 nm have been observed.
This indicates that superlatices with ultrashort periods can be achieved, using these
coherently strained stressors.

A.6. Self-assembled InAs dots/rings

The dynamics of tunneling into self-assembled InAs
dots

Self-assembled strained islands have attracted particular
attention as they provide for well-defined, nanometer-size quantum dots with sizes
in the 10 nm range. These systems are of great interest, not only for studying the
basic properties of man-made "artificial atoms", but also because of possible
device applications. Using capacitance and far-infrared spectroscopy, the many-particle
ground states as well as the excitations of the dots have been investigated in the
past. For possible implementation of InAs dots in future devices, it is crucial to
understand the charging dynamics of these structures. We investigated the dynamics
of tunneling from a three-dimensional back contact into ensembles of self-assembled
InAs quantum dots by frequency-dependent capacitance spectroscopy. Apart from the
influence of the tunneling barrier thickness, we investigated the influences of Coulomb
blockade, magnetic fields and temperature. An equivalent resistance-capacitance (RC)
circuit can be derived from the balance of charge in the device which allows us to
determine the tunneling times for each state of the dots. The different tunneling
times for different many-particle states are explained by a reduced tunneling barrier
and Coulomb interaction. A magnetic field applied perpendicular to the tunneling
direction results in a strong suppression of the charging signal, which is attributed
to enhanced localization caused by the magnetic field. Calculations for 3D-0D magneto-tunneling
can account for the experimental data. Our investigations show that by suitable sample
design the tunneling time can be adjusted in a wide range which promises technical
applications ranging from charge storage to ultrafast switching.

Self assembled InAs rings

Modification of the growth conditions during the fabrication
of InAs-Islands can be used to create InAs-InAs-nanorings instead of dots (see fig.
below). In order to demonstrate that the morphology of InAs-quantum rings like in
fig. 1a translates into a ring-like electronic structure we have investigated the
many-body ground states and their excitations by both capacitance and far infrared
spectroscopy. These experiments show that the electronic properties are different
than for dots. In particular we have observed a magnetic field induced transition
of the one electron ground state and additional resonances in the excitations (compared
to the two mode spectrum of quantum dots). Our results can be qualitatively explained
by theoretical calculations assuming a parabolic potential as in the fig below.

Passivation by thermal oxidizing of the silicon structures,
deposition of a thin gate oxide on top of the structure and evaporation of a metallic
top-gate completes the preparation process of this SET-devices.

In the above figure, the Coulomb blockade oscillations
and the Coulomb blockade diamond observed in a highly doped SOI-nanowire at 4.2 K
are shown.

Temperature dependence of the Coulomb-blockade
oscillations in a quasi-metallic silicon nanowire

The figure shows the temperature dependence of the top-gate controlled Coulomb
blockade oscillations in an SET-transistor realized in a highly doped SOI-Quantum
wire.

A. 8. Micromachined silicon devices and nanostructures

Underetching of silicon nanostructures on SOI-devices
suspends the device by removing the buried oxide under the silicon film. Using this
technique, we succeed in fabricating suspended, highly doped silicon nanowires and
quantum dots with lateral dimensions down to 50 nm. These devices show clear deviations
from ohmic behaviour. We expect to observe Coulomb-blockade on these devices in the
near future, allowing also the study of electron-phonon interaction in this regime.
Especially the application as a high power device seems to be obvious, since the
electron-phonon interaction, leading to unwanted heat consumption, is strongly suppressed
due to selection rules for scattering of electrons and acoustic phonons.

Furthermore, applying the technique of epitaxial-liftoff,
that is well established at our institute for GaAs/AlGaAs-structures, to the SOI-system,
we built a first thin-film transistor on a 190 nm thick single-crystalline silicon
film, that was removed from the SOI-substrate by underetching and stuck to a quartz
substrate. The figure below shows the I-V characterisitic of this novel MOSFET structure.
This technology promises applications in thin-film high-quality MOS-devices on arbitrary,
especially fexible or curved substrates.

I-V characteristics of the bonded MOSFET

MOSFET in a thin single crystalline
silicon film attached to quartz.

A.9. Nanomechanics

The basic idea of most sensors, such as acceleration or gas sensors,
is to detect the change in the mechanical motion of a small structure electrically.
Scaling down these devices to dimensions of only a few 100 nm is important for increasing
speed as well as sensitivity. Moreover, studying the mechanical properties of such
devices reveals interesting physics as well.

In the figure below you can see a small beam, machined out of silicon and gold. The
fabrication technique is as follows: The basic material is a conventional Silicon-on-Insulator
(SOI) substrate. First the structure is written with a Scanning Electron Microscope
(SEM). In order to suspend the beam two etching steps are required, first dry etching,
performed with an Reactive Ion Etcher (RIE), and second, wet etching, performed in
diluted HF.

The sample is cooled down to 4.2 K and a magnetic field of up to 12 Tesla is applied,
which is in the plane of the sample, but perpendicular to the beam. A high frequency
current is driven through the beam, resulting in an inductive force out of the plane
of the sample. The motion of the beam induces an electromotive force, which results
in an increased absorption of electrical power. In our experimental setup, we are
measuring the reflection of the signal applied. By changing the excitation frequency,
we are able to detect the mechanical eigenfrequency of the beam. Low power excitation
leads to a symmetric resonance curve, i.e. the resonator is in the linear regime.
If the power of the excitation is increased at B = 12 T, the resonator can be driven
into nonlinear response. The shape of the absorption peak is then strongly distorted
and a region with infinite derivative appears.

The region of infinite derivative can be used for charge detection. The gate nearby
can be charged by a negative voltage. This voltage shifts the position of the eigenfrequency
of the beam and thus the position of this region. This shift can be used to determine
the charge on the gate, if the charge fluctuation on the gate is minimized. In the
present structure an accuracy of approximately 70 electrons was determined.

One familiar example of a device, which is working according to the principle of
combining electrical and mechanical properties, is the classical bell. When scaling
down such a bell, it should be possible to count electrons, which flow during each
cycle of motion. In the figure below a typical structure used in the experiments
is shown. A suspended, completely metallized clapper C, source-drain contacts S and
D and two gates G1 and G2 are seen, which are used to drive the clapper electrostatically.
Again, by changing the frequency of the excitation, the eigenfrequencies of the clapper
can be determined. Due to the more complex geometry, the mechanical resonance spectrum
shows response at much more frequencies. Via the simple relation I = nef (f: frequency,
I: current, e: electron charge), the number n of electrons transferred in each cycle
can be determined. By reducing the power of the excitation, we are able to transfer
at least 7 ± 2 electrons per cycle. The reduction to one electron per cycle
could be possible, if Coulomb blockade effects at low temperatures are included.

B. INTRA- AND INTERBAND SPECTROSCOPY

Interband absorption spectra of an array of self-assembled quantum dots. With
increasing carrier density in the dots the energetically low-lying transitions
become 'Pauli-blocked' as the final states for the transitions are already occupied
by electrons. Note the extreme sensitivity of the experiment as indicated by the
very small absorption measured !

B.1. Electron Systems in Band Gap Engineered Quantum Systems

Modern crystal growth techniques like molecular beam epitaxy offer
the unique advantage of tailoring the band edges of different semiconductor systems
in a very controlled manner. A prominent example is the so-called parabolic quantum well (PQW), but also more complex
systems can be realized.

In close co-operation with the research group of A.C. Gossard in Santa Barbara we concentrate on the investigation
of the collective reponse of low-dimensional electron systems in such man-made semiconductor
structures.In 1998, we focused on the investigations of the possibility to emitt
far infrared radiation from such structures. As has been demonstrated by the group
of Prof. Gossard, the emission from a parabolic quantum well can be triggered by
a strong heating of the electron system by application of a large drift current to
the carriers. The frequency of the emitted radiation nicely corresponds to the one
of optical absorption, hence also obeying Kohn's Theorem. We followed an alternative
route, namely to impose the strong lateral electric fields of a piezoactive surface
acoustic wave onto the electron system in a PQW. Here, we also employed the hybridization
technique mentioned above, where the SAW is excited on a LiNbO3 substrate and the
semiconductor quantum well is deposited on top in form of a thin film.

B.2. Spectroscopy of Self-Organized InAs Quantum Dots

With decreasing sizes of nanostructures the problem of homogeneity
becomes more and more important. Ideally, one would like to study large arrays of
identical quantum systems. One elegant way to achieve this is to use fabrication
mechnisms where the shape and the dimensions of the nanostructures are given by,
energetic considerations, such that energy minimization will lead to the desired
sample homogeneity. This happens, e.g. in the Stranski-Krastanow growth mode of InAs
on GaAs heteroepitaxy and results in layers of uniform quantum dots of ~ 20 nm diameter
and ~ 7 nm height. In a close collaboration with P. Petroff 's group at UC Santa Barbara we have demonstrated that these
dots can be integrated into a metal-insulator-semiconductor heterostructure, which
allows us to tune the electron number per dot, determine it by capacitance spectroscopy, and study the dots'
excitation in the far-infrared. Here, the far-infrared spectroscopy resembles the
atomic spectroscopy mentioned in the introduction. Indeed, we can "tune the
dots through the table of elements" and distinguish, e.g. "quantum dot
Helium" and "quantum dot Lithium" by their excitation spectrum. Furthermore,
we can directly compare the different ground states of these few-electron systems,
and, from a comparison between ground state and excitation energies, derive detailed
information on the Coulomb and quantization contributions to the energetic structure
of these man-made "atoms". A natural step further is the fabrication of
"quantum dot molecules". This can be achieved because the Stranski-Krastanow
islands tend to align when they are grown on top of each other. So far, no direct
evidence for quantum mechanical coupling was observed, since the distance of the
dots (10-20 nm) does not allow for a considerable overlap of the wave functions.
However, the Coulomb interaction between the carriers in the different dot layers
reveals itself in a distict shift of the many particle ground state energy. For the
closely (10 nm) spaced dots, the charging sequence can be affected by an applied
magnetic field.

In 1998, we continued our research on voltage - controlled lateral
superlattices to demonstrate the trapping of photogenerated excitons. We were able
to show that such a system may act as an efficient trap for neutral excitons and
investigated the generation-, diffusion and relaxation processes in great detail.Triggered by the success of the 'photon
conveyor belt' described above, we also use static interdigitated
gates to efficiently trap photogenerated charges in this potential landscape. Here,
we can store optical information in an accumulative way in our laterally defined
staorage cells. After some accumulation time, the optical storage cell can be switched
to the 'storage mode' and finally be triggered externally to release the stored information
in a flash of light. Even for a not optimized system, storage times in excess of
50 musec have been already demonstrated!Presently, we are exploring possibilities to exploit these remarkable
effects in terms of new, alternative and superior optical devices for detection,
storage, switching, routing of optical signals as well as for optical signal processing
like pattern recognition etc.

B.4. Intraband and Interband optics on self-assembled quantum
dots

We have set up a system to measure the transmission coefficient
of a sample at wavelengths around 1 micron with extremely low noise for interband
experiments. The idea is to measure interband absorption through changes in the intensity
of transmitted light. We have applied the technique to samples containing self-assembled
quantum dots where the change in absorption is only about 1 part in 104
at resonance. Nevertheless, by using charge-tunable dots provided by P. Petroff 's group at UC Santa Barbara we have detected the transitions with good signal:noise.
The charge-tunable dots have the advantage that we can load the dots with a discrete
number of electrons and therefore measure the optical properties of charged excitons.
The results show how the various interband transitions disappear according to the
occupation of the dots (Pauli-blocking). Furthermore, there are energy shifts in
the higher transitions as we occupy the electron ground-state. We are presently extending
the work also to samples with two dot layers, and to dots provided by Harri Lipsanen,
Helsinki which are defined with stressors. The experimental techniques to study the
SAD optics are near field microscopy, Fourier-transfrom spectroscopy, and surface
acoustic wave transmission experiments.

C: NANOMETER FABRICATION AND CHARACTERIZATION

SEM micrograph of a double junction SQUID: The junctions are generated in the
evaporated Al at the optically predefined constrictions. The lower right inset shows
a magnification of one of the junctions -- it is clearly seen that all the material
is removed. The upper inset shows a schematic representation of the nano-plough:
the EBD tip isdeposited with an angle on a common AFM-tip, causing a vertical force
when dragged through material.[20-98]

C.1. New 'Supertips' for AFM

The quality of scanning probe microscope, in particular the atomic
force microscope (AFM) crucially depends on the quality of the tips used. The key
features defining the quality are the radius and the aspect ratio, i.e. the geometry
of the tip, as well as their durability.Over the last years we kept on developing the so called electron-beam-deposition
(EBD) further and further. Our EBD-tips combine high resolution imaging, which is
at least comparable to that known from the very best silicon AFM sensors, with the
extreme durability of diamond coated sensors. Furthermore they can be costum made
to almost every geometry requested, ranging from 100 nm short to well over 5 µm
long needles, with tip radii from a very few nanometers to some 100 nm. Even more
complex sensors, e.g. hook like tips, curved needles, closed loops to name but a
few, can be made upon request. Whatever the need for a tool on the nanometer lengthscale
might be, EBD material can well be the material one is looking for.

Close up view of the tip from the left.

The tip is deposited under such an angle,that it comes normal to the sample surface.Dimensions:

These tips are perfectly suited for non-destructive characterisation
and precise quantitative measurement of semiconductor nanostructures, in particular
"flying", free standing structures such as (Robert, Laura, Armin, Arthur
et al.) or deep trenches and holes.The feasability of the tips in biology, medicine, biophysics, material
science have been proven by a number of collaborations including the Deutsche Krebsforschungszentrum Heidelberg, Materialforschung
Jena, Olympus Inc., Carl Zeiss, VEECO, Max-Planck Institut für Kolloid- und
Grenzflächenforschung Berlin.As a spin-off these activities the company NanoTOOLS GmbH was founded, which makes these
tips commercially avaible.

C.2. Nano-ploughed Josephson-Junctions as on-chip Radiation
Sources

A new technique was developed which enables the fabrication of
highly transparent Josephson junctions in combination with mesoscopic devices. We
utilize a modified AFM tip to plough grooves into superconducting material, thus
defining a weak link [20-98].
In reversion to the voltage-standard, the ac-Josephson effect "converts"
an externaly applied voltage V to electromagnetic radiation of frequency f
according to the basic Josephson equation:

f = 2e V / h

Direct mechanical structuring seems not to be the obvious method
to build mesoscopic devices. Nevertheless, the main advantages of this approach are
the ease of integration of the different techniques and the flexibility to build
spectroscopic tools in the quantum limit. While conventional spectroscopy relies
on bulky external frequency sources, here the radiation source can be placed within
nanometer distance to the device under test. This not only allows the generation
of high intensity electromagnetic fields exactly at the nano-structure, minimizing
effects by external heating, but also spectroscopy on quantum structures and the
investigation of coherent electron tunneling phenomena directly in the frequency
domain.

SEM-micrograph of a double quantum dot with an embedded nano-ploughed weak-link.
The circle indicates the position of the variable thickness weak link, acting as
a Josephson Junction.

Schematic representation of the double dot from the left. Electron transport through
the system is made possible by absorption of photons.

In the SEM image above, the weak link is made within the superconducting
split-gates, which are used to electrostatically form a (conventional) double quantum
dot and serves as a source of millimeter wave radiation around 100~GHz. We find that
the millimeter wave emission of the weak link leads to a bolometric effect in the
case of quantum point contact spectroscopy.

C.3. Direct Fabrication of Nanostructures by AFM induced Oxidation

Anodic electrochemical oxidation of metal and semiconductor surfaces
is a well-established technique to grow oxides on them. For some materials, e.g.
GaAs, it is the only way to oxidise them. Hereby, the metal/semiconductor is placed
into an electrochemical cell, where it is biased as anode. Whereever it is exposed
to the electrolyte, an oxide will be formed.The electrochemical reaction can now be laterally confined to the
dimensions of an AFM probe. Under ambient conditions, a water meniscus will be formed,
which then acts as a nanometer scaled down version of the standard electorchemical
set up. Again, the formation of the oxide is constricted to the exposed areas - which
is now almost identical to the meniscus. Optimizing the relevant parameters such
as bias voltage, exposure time, humidity to name but a few, reducing the size of
the meniscus by using hydrophobic probes and compensating the electrostatic forces
by working in non-contact AFM mode, the local oxidation becomes a lithographic technique
allowing to define 2D, 1D and 0D insulating regions [13-98].

Oxide structures, defined by AFM induced local oxidation.
Left: Ti island, 18 x 24 nm2, seperated by TiO tunneling barriers from
the Ti leads.
Right: suqare oxide lattice on top of a InAs/AlSb surface quantum well. Here the
oxide forms an antidot lattice with a period of 80 nm.

C.4. X-ray investigations of laterally structured surfaces

For a collaboration with M. Tolan and W. Press at Kiel University we fabricate surface gratings
on silicon (period: 500 nm to 1000 nm; height: 1 nm to 50 nm) using holographic lithography
followed by a dry etching process [4-98]. Subsequently such gratings are covered with a thin deposit of different
materials and studied by small-angle X-ray reflection from this well defined surface
roughness. For the case of polymers, e.g. polystyrene or PMMA the propagation of
surface roughness from the patterned silicon surface was measured by atomic force
microscopy and synchroton x-ray reflection. The decay length of the surface modulation
was much longer than that observed in simple liquids. By measuring the time dependence
of the surface corrugation amplitude we are able to extract the surface diffusion
coefficient.

A beautiful example for another successful co-operationbetween
our group and the one led by Prof. J. Peisl is the investigation of Surface
Acoustic Waves using X-ray techniques. By phase-locking a synchrotron to a SAW device
(!), we were able to spatially resolve the SAW induced lattice deformations on various
substrates. Using this exciting technique, we study for instance new transducer concepts
including focussing sound generators.

X-Ray representation of a GaAs sample with a surface acoustic wave present.

Also self-assembled dots have been studied using
grazing-incidence x-ray diffraction. The small size-dispersion and the crystalline
quality of the dots allowed for a detailed investigation of the SHAPE, SIZE, and
LATTICE CONSTANT of these self-organized nanostructures. Loosely speaking, it has
been possible (by diffraction techniques!) to "slice" the dots into thin
layers and obtain the above quantities for each of these layers. The morphological
information obtained by this "nanotomography", which cannot be gained from
other techniques, is now available to test possible growth scenarios and shed light
on the complex dot formation.

C.5. Low-energy electron-beam lithography

Using low-energy electrons for our ebeam-lithography
and the negative electron resist calixarene, we realize lateral structure sizes down
to 10 nm and even smaller. We show, that this resolution limit is achieved even with
electron energies of only 2 keV. Proximity effects occuring in the regime around
40 keV are practically ruled out at these low acceleration voltages.

In addition, we show, that radiation damage on high-mobility
2DES realized in a semiconductor heterostructure is of no importance for electron
energies up to 20 keV allowing the application for high-mobility (HEMT)-devices.

Using calixarene as an etching mask we fabricate nanowires
and quantum dots in Silicon-on-Insulator (SOI) films. The following figure shows
an example of such a structure.

Axel Lorke
"Many-Particle Ground States and Excitations in Nanometer-Size Quantum Structures"
13th International Confernce on High Magnetic Fields in Semiconductor Physics
(Nijmegen, The Netherlands, 1998)